Secondary batteries are used as portable power sources for a variety of personal electronic applications, such as mobile phones, laptop computers, cordless power tools, digital cameras, digital camcorders, hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV) and electric vehicles (EV). To support these applications, secondary batteries based upon lithium have emerged as the predominate solution. Unfortunately, a number of the chemistries used in these secondary batteries are prone to thermal run-away, up to and including, battery fires. For instance, a 1-in-200,000 failure rate triggered a recall of almost six million lithium-ion packs used in laptops. As the energy storage requirements for hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV), or electric vehicle (EV) applications is orders of magnitude higher than for a laptop, the need for a solution to the thermal run-away and potential fire dangers dramatically increase.
The root cause of the thermal run-away is believed to be an internal short circuit across the battery separator. Once an internal short circuit across the battery separator forms, an electrical current begins to flow through this short circuit and begins to heat the chemical components of the battery. As the temperature inside the battery rises, the higher temperatures trigger chemical reactions that lead to additional heat generation and to still higher temperatures. If unchecked, this process may continue until the materials of the battery are combusting, and pose a dangerous situation to the users of these batteries.
To address the thermal run-away issues of secondary batteries, several solutions have been posited that call for a change in the electrochemical reactions of the secondary battery. Unfortunately, such changes typically sacrifice the total energy storage capability of the secondary battery. For instance, replacement of the graphite negative electrode in a lithium ion battery with a lithium titanate electrode, while effective in improving the safety, results in a significant decrease in the theoretical gravimetric capacity from 372 mAh/g to 175 mAh/g. In other solutions, flame retardants have been introduced to battery electrolytes, however, while flame retardants help to restrain fire, they do not prevent the fire. Hence, such solutions have limited effect in preventing fire. In another solution, battery cell separators have been coated with ceramics to strengthen the separator against breaches which can lead to internal short circuiting. Such an approach may limit the number of internal short circuits, but it does little to inhibit internal short circuits caused by impurities in the electrolyte that form dendrites from one electrode through the separator and to the other electrode. Thus, the need for an approach to solve the safety issues in lithium- and lithium-ion batteries persists.